The present invention relates to a process and system for the production of olefins and particularly to processing the charge gas feed to more effectively recover the product and process the by-products.
Ethylene, propylene and other valuable petrochemicals are produced by the thermal cracking of a variety of hydrocarbon feedstocks ranging from ethane to vacuum gas oils. In the thermal cracking of these feedstocks, a wide variety of products are produced ranging from hydrogen to pyrolysis fuel oil. The effluent from the cracking step, commonly called charge gas or cracked gas, is made up of this full range of materials which must then be separated (fractionated) into various product and by-product streams followed by reaction (hydrogenation) of at least some of the unsaturated by-products.
The typical charge gas stream, in addition to the desired products of ethylene and propylene, contains C2 acetylenes, C3 acetylenes and dienes and C4 and heavier acetylenes, dienes and olefins as well as a significant quantity of hydrogen and methane. Aromatic as well as other ring compounds and saturated hydrocarbons are also present.
In U.S. Pat. No. 5,679,241 and U.S. patent application Ser. No. 10/202,702, filed Jul. 24, 2002, ethylene plant front-end catalytic distillation column systems are disclosed in which the highly unsaturated hydrocarbons, as acetylenes and dienes, are reacted with the contained hydrogen in the steam cracker charge gas compressor train to form olefins. In the process, it is desired to control the catalyst bed temperatures to as high a level as possible consistent with a low fouling rate. This maximum temperature minimizes the quantity of catalyst required. It can also increase overall selectivity to ethylene and propylene. The conditions that achieve the optimum catalytic distillation catalyst temperature can, however, result in a column bottoms temperature that is relatively high and can increase the fouling rate in the bottom of the column. While this fouling rate can be controlled by adding inhibitors, it is desirable to design the catalytic distillation hydrogenation system to achieve high catalyst bed temperatures while maintaining a low bottoms temperature and a low core fouling rate in the column system.